Depending on the degree of turbulence of the flow, the value of the pressure gradient in the direction of flow, and the value of Reynolds number Re.sub.2 (a dimensionless parameter which is a function of the chord length l of the blade profile, the exit velocity c.sub.2, and the kinematic viscosity .nu..sub.2 at the outlet of the blade passage) the laminar boundary layer is converted into a turbulent state either directly or with eddy formation of transition or separation bubbles.
In low-pressure turbines, for instance in the output stages of axial flow gas turbines of aircraft, the value of Reynolds number is between 50,000 and 500,000. This magnitude of Reynolds number lies within the transition region between laminar and turbulent flow and thus decelerated laminar boundary layers change their state to turbulent flow with the formation of transition or separation bubbles and produce large inherent losses.
Up to now, it has been assumed that in multi-stage turbines there is such a high degree of turbulence that laminar flow is either entirely impossible or else the boundary-layer transition upon deceleration in flow takes place directly, i.e. without eddy formation of bubbles. Recent investigations have shown, in contradiction to this that in the region of accelerated flow in low-pressure turbines the laminar boundary layer condition may be maintained but that the efficiency of the energy transfer is considerably impaired by flow losses related to the transition between laminar and turbulent flow and the formation of separation bubbles.
From the specialized literature on airfoil aerodynamics it is known to use mechanical turbulators such as trip wires, protruding edges, steps or similar obstacles in order to avoid production of separation bubbles upon transition from laminar boundary layer flow to turbulent flow. It has been found that the provision of such turbulators on the surface of the airfoil profile increases the coefficient of drag of the profile and can lead to considerable losses, particularly at higher values of Reynolds numbers, i.e. in a region in which the turbulators no longer directly fulfill their actual purpose, namely to accelerate the transition of the boundary layer from laminar flow to turbulent flow without the formation of bubbles.
In a technical report by Francis R. Hama, James D. Long and John C. Hegarty of August 1956 entitled "On Transition from Laminar to Turbulent Flow," published by Document Service Center of Dayton, Ohio, USA, water tank tests on the transition from laminar to turbulent boundary layer flow on a flat plate are described and it is suggested that a corrugated wire or a row of thin triangular plates which are bonded on a flat plate can be used to bring about the transition from laminar to turbulent boundary layer flow. No disclosure is given in this publication regarding aerodynamic blade wheels.
Mechanical turbulators have, up to now, not been used in turbomachine construction. Rather, in DE-PS No. 30 43 567, there is disclosed an arrangement for controlling the boundary layer flow on aerodynamic profiles in which a rapid transition to turbulence is obtained by blowing fluid out in the region of separation of the laminar flow, thus preventing the occurrence of laminar separation bubbles. Such an arrangement has the advantage that no obstacles are present on the airfoil profile within the range of operation at high values of Reynolds numbers within which laminar separation bubbles no longer occur anyway and thus no additional losses need be tolerated. However, one disadvantage of this arrangement is that the cost of manufacture of an airfoil profile with it is very high. For blade wheels of turbomachines, particularly in the case of thermal turbomachines, the arrangement in DE-PS No. 30 43 567 is unusable, or only usuable to a very limited extent, since as a result of the high mechanical and thermal stressing of the blades, the provision of corresponding flow channels for the fluid to be blown out would unacceptably weaken the blades.